The peninsula, the easternmost "leg" of the larger Halkidiki peninsula, protrudes 50 kilometres (31 mi)  into the Aegean Sea at a width of between 7 and 12 kilometres (4.3 and 7.5 mi) and covers an area of 335.637 square kilometres (129.59 sq mi). The actual Mount Athos has steep, densely forested slopes reaching up to 2,033 metres (6,670 ft). The surrounding seas, especially at the end of the peninsula, can be dangerous. In ancient Greek history two fleet disasters in the area are recorded: In 492 BC Darius, the king of Persia, lost 300 ships under general Mardonius (Herodotus "Histories" book VI (Erato), Aeschylus "The Persians"). In 411 BC the Spartans lost a fleet of 50 ships under admiral Epicleas. (Diodorus Siculus, "Bibliotheca historica" XIII 41, 1–3).

Athos in Greek mythology is the name of one of the Gigantes that challenged the Greek gods during the Gigantomachia. Athos threw a massive rock against Poseidon which fell in the Aegean sea and became the Mount Athos. According to another version of the story, Poseidon used the mountain to bury the defeated giant.

Homer mentions the mountain Athos in the Iliad (Rhapsody Ξ 219). Herodotus mentions the peninsula, then called Acte or Akte, telling us that Pelasgians from the island of Lemnos populated it and naming five cities thereon, Sane, Cleonae(Kleonai), Thyssos (Thyssus), Olophyxos (Olophyxis), Acrothoï (Akrothoön). (Herodotus, VII:22) Strabo also mentions the city of Dion (Dium) and that Acrothoï is near the crest. (Strabo, Geography, VII:33:1) Eretria also established colonies on Acte. At least another city was established in the Classical period: Acanthus (Akanthos). Some of these cities minted their own coins.

The peninsula was on the invasion route of Xerxes I, who spent three years excavating a channel across the isthmus to allow the passage of his invasion fleet in 483 BC. After the death of Alexander the Great, the architect Dinocrates(Deinokrates) proposed to carve the entire mountain into a statue of Alexander.

The history of the peninsula during latter ages is shrouded by the lack of historical accounts. Archaeologists have not been able to determine the exact location of the cities reported by Strabo. It is believed that they must have been deserted when Athos' new inhabitants, the monks, started arriving at some time before the 7th century AD.

Though land-linked, Mount Athos is practically accessible only by boat. There are two large ferries, Agios Panteleimon and Axion Estin, that travel daily (weather permitting) between Ouranoupolis and Dafni, with stops at some monasteries on the western coast along the way. There is also a smaller speed boat, Agia Anna, which travels the same route, but with no intermediate stops. It is possible to travel by ferry to and from Ierissos for direct access to monasteries along the eastern coast. The daily number of visitors entering Mount Athos is restricted and all are required to obtain a special entrance permit valid for a limited period. Only males are allowed entrance into Mount Athos, which is called "Garden of the Virgin" by monks,and Orthodox Christians take precedence in the permit issuance procedure. Only males over the age of 18 who are members of the Eastern Orthodox Church are allowed to live on Athos, either as monks or as workers.